The objective of the studies proposed in this application is to identify genotypic combinations of mice and mycobacteria that result in significant alterations of host responses to experimental mycobacterial infection. Susceptibility or resistance to experimental infection will be defined by determination of median survival time, weight loss, mycobacterial load in the lung and spleen, and in addition various immunological parameters in lung and other tissues. In these experiments, the host genome will be varied by use of 37 recombinant congenic strains (RCS), which are derived from inbred progenitors that are either susceptible to tuberculosis (A/J, abbreviated A) or resistant to tuberculosis (C57BL/6J, abbreviated B). The AcB/BcA RCS are now fully inbred and genotyped with a dense set of genome-wide microsatellite markers. These strains will be infected with a panel of Mycobacterium tuberculosis and M. bovis strains that have been deleted for genome regions associated with attenuation of BCG vaccines. The infection protocol in the RCS will reveal informative, significant deviations from the "expected" or "parental" disease phenotypes which signify the presence of quantitative trait loci (QTL) with strong effect on phenotype expression and possibly specific gene(s) interaction between the host and pathogen. In RCS carrying QTL with strong effect on phenotype expression, changes in gene expression level in both lung macrophages and intracellular mycobacteria will be revealed by microarray analysis. The knowledge to what extent host responses in mycobacterial infections are a reflection of specific host pathogen combinations will be crucial for our understanding of the epidemiological flow of M. tuberculosis through an exposed population.